Hydrogen Storage Composition

ABSTRACT

A hydrogen storage composition comprises a particulate alloy comprising grains of magnesium, wherein the grain boundaries contain phases comprising nickel and at least one non-nickel transition metal, wherein the nickel is present at levels of ≦5 wt % based on the composition as a whole, and wherein the at least one non-nickel transition metal is present at levels of ≦0.5 wt % based on the composition as a whole.

This invention relates to a hydrogen storage composition and inparticular to a hydrogen storage composition comprising a particulatealloy comprising magnesium and nickel.

Hydrogen has great potential as an energy source because it can beeasily generated from renewable sources and its use creates neither airpollution nor greenhouse gas emissions. The principal problem with theuse of hydrogen is that as a gas it is difficult to store, it may bestored in vessels as a cryogenic liquid or as a compressed gas but suchstorage vessels are not well suited to everyday use.

An alternative means of releasably storing hydrogen over a plurality ofcycles is to use a metal or an alloy, where the metal or alloy absorbsand stores relatively large amounts of hydrogen by bonding with thehydrogen to form a hydride. These materials have the ability to absorband release hydrogen without deteriorating. The rate of absorption anddesorption can be controlled by temperature and pressure. The UnitedStates Department of Energy (DoE) have set a target (to be achieved by2010), by which hydrogen storage materials may be judged for theirpotential practicable value. This target includes a storage capacity of6 wt % (see Hydrogen Storage: The Key Challenge Facing a HydrogenEconomy by Motyka et al., published in March 2004, available from the USOffice of Scientific and Technical Information).

Magnesium itself has a relatively high capacity for hydrogen storage, inthe form of magnesium hydride, but unfortunately the reaction kineticsfor the absorption and desorption of hydrogen are too slow. Anadditional problem is that hydride formation at the surface of themagnesium particles can block further diffusion of hydrogen into thebulk of the material.

In prior proposals, it was realised that increasing the surface area ofthe magnesium particles could improve the hydrogen absorption anddesorption characteristics and thus different techniques were tried inan effort to achieve this.

High energy ball milling has been used to create mechanical alloys ofmagnesium of significantly smaller particle size. This techniqueintroduces large numbers of dislocations within the metal crystals withlarge areas of grain (or crystallite) boundaries and high levels ofinternal defects. These grain boundaries act as paths for the fastdiffusion of hydrogen and reduce the volume of material that is accessedby one path, such that hydrogen absorption and desorption times aresubstantially reduced.

The ball milling technique is a popular approach to improving thehydrogen storage characteristics of magnesium, and many publicationsstudy this process and how varying the conditions of the process mayinfluence the final material (e.g. Huot et al., J. Alloy. Compd., 1995,231, 1-2, p. 815-819; Schulz et al., Mat. Sci. Eng. A-Struct., 1999,A267, 2, p. 240-245; Bobet et al., Mater. Manuf. Process, 2002, 17, 3,p. 351-361). Another focus of hydrogen storage research has been todetermine how additives may influence the properties of magnesiumhydrides, possibly by catalysing the dissociation of H₂ molecules at theabsorbing surface. This work has tended to concentrate on cobalt, ironand nickel, which are mechanically alloyed with the magnesium during theball milling stage (Bobet et al., Int. J. Hydrogen Energy, 2000, 25, 10,p. 987-996). Recently we have shown that ball milling of at least onereducible platinum group metal (PGM) compound with magnesium, for afraction of the overall milling time, results in nanoparticles of PGMsbeing located predominantly at the surface of the milled particles (WO2004/016817). The addition of nanoparticles of PGMs was shown to improvethe hydrogen absorption/desorption characteristics of the milledmaterial.

Another approach to improve hydrogen storage capacity andabsorption/desorption kinetics was proposed by Friedlmeier et al., whoprepared a magnesium nickel alloy by melting the two metals together,then grinding the alloy to produce small particles (Int. J. HydrogenEnergy, 1988, 13, 8, p. 467-474).

GB 524,113 and GB 596,102 describe magnesium alloys that may containnickel and other transition metals. The alloys are intended forstructural purposes and may be cast, forged, extruded or pressed to formarticles such as engine parts.

However, although extensive studies on metal substitution and millingtechniques have been carried out on alloys suitable for hydrogen storageto date, nothing has yet been developed to reliably meet all therequirements set down by the DoE.

We have now found, very surprisingly, that a magnesium-containing alloy,comprising ≦5 wt % nickel and ≦0.5 wt % of at least one non-nickeltransition metal, prepared by melting the magnesium, nickel and the atleast one non-nickel transition metal is a more active hydrogen storagematerial than prior art alloys containing significantly more nickel. Webelieve that this is due to the unusual structure of the alloy producedwhen the alloy is cooled from the melt, wherein the internal structureof the solid comprises grains (or crystallites) of magnesium surroundedby grain boundaries of a different chemical composition (i.e. made up ofa different phase). The chemical composition of the grain boundaries isrich in the nickel and the at least one non-nickel transition metal incomparison with the chemical composition of the grains themselves.

According to one aspect, the invention provides a hydrogen storagecomposition comprising a particulate alloy comprising grains ofmagnesium, wherein the grain boundaries contain phases comprising ofnickel and at least one non-nickel transition metal, and wherein thenickel is present at levels of ≦5 wt % based on the composition as awhole, and wherein the at least one non-nickel transition metal ispresent at levels of ≦0.5% based on the composition as a whole. Thegrain boundaries are present within the internal structure of theparticles, as well as forming the surface of the solid particle, andtherefore are not located solely at the surface of the solid.

Usefully, we have found that the proportion of nickel present can beless than 2 wt %, or even as low as from 0.01 wt % to 1.00 wt %, withoutsignificantly impacting upon hydrogen uptake and storagecharacteristics. Additionally, we have found that the proportion of theat least one non-nickel transition metal present may be at levels as lowas from 0.01 wt % to 0.50 wt %, commonly between 0.01 wt % and 0.20 wt%, without significantly impacting upon hydrogen uptake and storagecharacteristics.

In one embodiment, the composition comprises particles of the alloy ofan average size of less than 500 μm, commonly less than 50 μm or even assmall as 500 nm.

Whereas the composition, as described, includes grains of magnesiummetal, it will be appreciated that, in use, this metal will be in theform of a magnesium compound, particularly magnesium hydride. Therefore,in one embodiment, the composition comprises the magnesium in the formof a hydride.

The at least one non-nickel transition metal can be selected from thegroup consisting of Pt, Pd, Ru, Ir, Ag, Au, Cu, Co, W and mixtures ofany two or more thereof. In one embodiment, the non-nickel transitionmetal is at least one of Pt, Pd, Ru, Ir, Ag or a mixture of any two ormore thereof. The presence of the at least one non-nickel transitionmetal aids hydrogen absorption, but it need not add significantly to theexpense of the composition as relatively small levels of the metals areneeded to achieve an advantageous improvement in activity.

According to a further aspect, the invention provides a method of makinga composition, according to the invention, wherein the magnesium, nickeland the at least one non-nickel transition metal are melted to form analloy. This alloy will then be cooled, preferably at a high cooling rate(for example ≧50° C./s for at least the first two seconds), to producesolid alloy, and then processed to produce particles of the desiredsize.

This processing step commonly involves casting of the molten alloy,although the molten alloy may be cooled and processed simultaneously bypowder spray atomisation to form a solid alloy of reduced particle size(in comparison to that formed by casting alone). If additionalprocessing is necessary to reduce the particle size of the solid alloyto less than 500 μm, then this may be carried out by grinding the solidalloy to produce coarse particles. Additionally the alloy particles maybe milled to produce particles of an even smaller crystallite size withhigh surface area, as desired, preferably under hydrogen to produce thehydride phase suited for use as a storage material.

According to another aspect, the invention provides an apparatus forproviding, on demand, a replenishable supply of hydrogen, whichapparatus comprising:

-   -   (i) a container comprising a hydrogen storage composition        according to the invention;    -   (ii) means for adjusting the pressure within the container;    -   (iii) means for adjusting the temperature of the contents of the        container; and    -   (iv) means, in use, for controlling the pressure adjusting means        and/or the temperature adjusting means.

Such an apparatus may have hydrogen storage capacities in excess of 6 wt% and 60 Kg/m³ based on a powder packing density of 1 g/cm³. Oneadvantage of this apparatus is that the control means controls thetemperature adjusting means to heat the hydrogen storage composition totemperatures of from 100 to 350° C., thereby to absorb or desorbhydrogen in the hydrogen storage composition. Additionally, the controlmeans controls the pressure adjusting means to adjust the pressure inthe container to pressures of from 1 to 10 bar (10⁻² to 10⁻¹ kPa),thereby to absorb or desorb hydrogen therein. Generally, hydrogenabsorption occurs at a lower temperature and higher pressure thanhydrogen desorption.

The pressure adjusting means may comprise a valve and/or a pump toenable hydrogen to be released from the container and supplied to arelevant application, e.g. a fuel cell, at an appropriate pressure; andfor the container and, hence, the composition to be replenished withhydrogen from a suitable source.

In one embodiment, the invention provides a mobile power sourcecomprising an apparatus as described above, whereas in anotherembodiment the invention provides a stationary power source comprisingan apparatus as described above.

According to one aspect, the invention provides a method of providing,on demand, a replenishable supply of hydrogen, which method comprisingproviding a composition according to the invention, comprising amagnesium hydride, in a temperature range of from 100 to 350° C. and apressure range of from 1 to 10 bar (10⁻² to 10⁻¹ kPa), thereby to desorbhydrogen absorbed therein.

In order that the invention may be more fully understood an apparatusembodiment and the following Examples are provided by way ofillustration only and with reference to the accompanying drawings, inwhich:

FIG. 1 is a graph of hydrogen absorption from a particulate Mg/1 wt %Ni/0.2 wt % Pd composition, prepared according to the method describedin Example 1, at temperatures ranging from 100° C. to 300° C.;

FIG. 2 is a graph of hydrogen desorption from a particulate Mg/1 wt %Ni/0.2 wt % Pd composition, prepared according to the method describedin Example 1, at temperatures ranging from 250° C. to 350° C.;

FIG. 3 is a graph of hydrogen absorption from a particulate Mg/1 wt %Ni/0.2 wt % Pd composition, prepared according to the method describedin Example 1, at temperatures ranging from 250° C. to 300° C., afterfrom 1 to 11 absorption/desorption cycles;

FIG. 4 is a graph of hydrogen desorption from a particulate Mg/1 wt %Ni/0.2 wt % Pd composition, prepared according to the method describedin Example 1, at 300° C., after from 1 to 9 absorption/desorptioncycles;

FIG. 5 is an electron microscopy image showing a particulate magnesiumalloy, prepared according to the method described in Example 1, withnickel and palladium-rich grain boundaries;

FIG. 6 is an electron probe microanalysis image, collected using X-rayfluorescence to highlight the presence of magnesium, of a compositionprepared according to the method described in Example 1;

FIG. 7 is an electron probe microanalysis image, collected (on the samearea of the same sample as FIG. 6) using X-ray fluorescence to highlightthe presence of nickel;

FIG. 8 is an electron microscopy image, collected (on the same area ofthe same sample as FIG. 6) using X-ray fluorescence to highlight thepresence of palladium;

FIG. 9 is a comparative graph of hydrogen absorption from a milled Mg/1wt % Ni/0.5 wt % Pd composition, prepared according to the methoddescribed in Example 2, at 300° C. after from 1 to 4absorption/desorption cycles; and

FIG. 10 is a schematic diagram of an apparatus embodiment according tothe invention.

Referring to FIG. 10, apparatus 10 comprises a container 12 containing ahydrogen storage composition 14, valve means 16 for adjusting thepressure within the container, heating means 18, comprising a heatingelement, for adjusting the temperature of the contents of the container,a power supply 19 for the temperature adjusting means and means 20, inuse, for controlling the pressure adjusting means and/or the temperatureadjusting means.

In use, a hydrogen demand input is made to control means 20 for a supplyof hydrogen. Control means 20 directs valve means 16 to open to enablehydrogen supply (thereby releasing pressure in the container therebypromoting hydrogen release from composition 14) and to activate heatingmeans to increase the temperature of hydrogen storage composition 14,e.g. towards 300° C., thereby to promote release of hydrogen. Thehydrogen demand input can be controlled by suitable open-loop feedbackmeans (not shown) in order to control the rate of hydrogen supplyrequired or to stop hydrogen supply completely as required.

In order to replenish a depleted amount of stored hydrogen incomposition 14, valve 16 is controlled to open to enable a high pressuresupply of hydrogen to be admitted to container and heating means 18 isdeactivated. In order to promote more rapid absorption of the hydrogenin composition 14, it may be desirable to provide refrigeration means(not shown) in order to more rapidly cool the composition, e.g. towards200° C., to promote absorption of hydrogen. However, such refrigerationmeans is unnecessary if the hydrogen supply is at low temperature.

EXAMPLE 1 Preparation of Mg/1 wt % Ni/0.2 wt % Pd by Melting and Milling

99.9 g magnesium and 1 g nickel together with 0.2 g palladium wereweighed out, (the excess of magnesium is present to compensate for theapproximate 1% vapour loss experienced during melting) and melted in apre-dried, heated Alumina crucible under 250-300 mbar (2.5×10⁻³ to3.0×10⁻³ kPa) Ar in a Balzers VSG002 vacuum melting furnace afterinitial evacuation at 10-5 torr. Melting time was approximately 10minutes, and the melt was kept in the molten state for the shortest timethat allowed complete dissolution and mixing, about 2 minutes. Themolten charge was then cast into a copper chill mould from approximately100° C. above the melting point. The recovered ingot was cleaned, andreduced to a crude powder under Ar by mechanical means before beingtransferred to an IFW Dresden for milling under hydrogen for >100 hours.

Electron microscope images for the composition are shown in FIGS. 5 to8. In particular, it can be seen in FIG. 6 that the magnesium isuniformly spread throughout the grains of the particulate magnesiumalloy because the grey shade indicates a high proportion of magnesiumwhilst the lighter shade represents the lesser amount of magnesium foundin the grain boundaries. FIGS. 7 and 8 demonstrate the presence ofnickel and palladium at the grain boundaries of the particulatemagnesium alloy as the dark areas indicate that very little nickel andpalladium is present within the grains whilst the grey areas show that agreater amount of these metals is present in the grain boundaries.

EXAMPLE 2 Preparation of Mg/1 wt % Ni 0.5% PGM by Milling Only

24.75 g magnesium hydride and 0.25 g nickel were weighed out and milledunder 3 bar (3×10⁻² kPa) hydrogen for 30 hours at 350 rpm, according toa 15 mins on/10 mins off cycle. The ball charge was 15×20 mm balls in a250 ml pot. 0.125 g of palladium nanoparticles were added for the lasthour of milling.

EXAMPLE 3 Hydrogen Absorption and Desorption Measurements

Hydrogen absorption and desorption measurements were carried on thecomposition of Example 1 using a Hiden Analytical IGA (IntelligentGravimetric Analyser) at varied temperatures. Hydrogen absorption wascarried out using H₂ gas at a pressure of 10 bar (10⁻¹ kPa). Hydrogendesorption was carried out using H₂ gas at a pressure of either 1 bar or50 mbar (1×10⁻² to 5×10⁻³ kPa).

The results of the IGA hydrogen adsorption and desorption measurementsare set out in FIGS. 1 and 2, which show that the composition of Example1 absorbs hydrogen to levels in excess of 6 wt % in less than 60 mins attemperatures as low as 250° C. and desorbs hydrogen fully within 30 minsat temperatures as low as 275° C. Additionally, FIGS. 3 and 4demonstrate that this composition continues to exhibit the favourablecharacteristics of readily absorbing and desorbing approx. 6 wt % ofhydrogen at temperatures ≦300° C. after multiple absorption/desorptioncycles, in contrast to the milled only composition shown in FIG. 9.

1. A hydrogen storage composition comprising: a particulate alloycomprising grains of magnesium having grain boundaries comprising phasesof nickel and at least one non-nickel transition metal, wherein thenickel is present in amounts of from 0.01 wt % to 5 wt % based on thecomposition as a whole, and wherein the at least one non-nickeltransition metal is present in amounts of from 0.01 wt % to 0.5 wt %based on the composition as a whole.
 2. The hydrogen storage compositionaccording to claim 1, wherein the particles of the alloy have an averagesize of less than 500 μm.
 3. The hydrogen storage composition accordingto claim 1, wherein the nickel is present in amounts of from 0.01 wt %to 2 wt % based on the composition as a whole.
 4. The hydrogen storagecomposition according to claim 3, wherein the nickel is present inamounts of from 0.01 wt % to 1.00 wt % based on the composition as awhole.
 5. The hydrogen storage composition according to claim 1, whereinthe at least one non-nickel transition metal is selected from the groupconsisting of: Pt, Pd, Ru, Ir, Ag, Au, Cu, Co, W and mixtures of any twoor more thereof.
 6. The hydrogen storage composition according to claim5, wherein the at least one non-nickel transition metal is selected fromthe group consisting of: Pt, Pd, Ru, Ir, Ag and mixtures of any two ormore thereof.
 7. The hydrogen storage composition according to claim 5,wherein the at least one non-nickel transition metal is present inamounts of from 0.01 wt % to 0.20 wt % based on the composition as awhole.
 8. The hydrogen storage composition according to claim 1, whereinthe magnesium alloy is in the form of a hydride.
 9. A method of making ahydrogen storage composition according to claim 1, comprising: meltingthe magnesium, nickel and the at least one non-nickel transition metalto form an alloy, cooling the molten alloy to produce a solid alloy andprocessing the solid alloy to produce particles of the desired size. 10.The method according to claim 9, wherein the molten alloy is cooled bycasting.
 11. The method according to claim 9, wherein the molten alloyis cooled and processed by powder spray atomisation.
 12. The methodaccording to claim 9, wherein the solid alloy is processed by grinding.13. The method according to claim 9, wherein the solid alloy isprocessed by milling.
 14. The method according to claim 13, wherein themilling is carried out under an atmosphere comprising hydrogen toproduce magnesium hydride.
 15. An apparatus for providing, on demand, areplenishable supply of hydrogen, said apparatus comprising: (i) acontainers comprising a hydrogen storage composition according to claim1; (ii) means for adjusting the pressure within the container; (iii)means for adjusting the temperature of the contents of the container;and (iv) means for controlling the pressure adjusting means and/or thetemperature adjusting means.
 16. The apparatus according to claim 15,wherein the temperature control means controls the temperature adjustingmeans to heat the hydrogen storage composition to temperatures of from100 to 350° C., thereby to absorb or desorb hydrogen in the hydrogenstorage composition.
 17. The apparatus according to claim 15, whereinthe pressure control means controls the pressure adjusting means toadjust the pressure in the container to pressures of from 1 to 10 bar(10⁻² to 10⁻¹ kPa), thereby to absorb or desorb hydrogen therein.18.-20. (canceled)